CLB ABOVE MAX CRUISE ALT TO AVOID TURB. PRESTALL BUFFET; EMER DSCNT.

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FLT PLAN TO FLY AT 37000 FT. THERE WAS MODERATE TURB AT THAT ALT OVER NM; SO WE CHOSE TO FLY AT 41000 FT. CENTER CLRED US TO THAT ALT. THE FMS SHOWED THAT THE OPTIMUM ALT WOULD HAVE BEEN ABOUT 40000 FT. I USED VERT SPD; 100 FPM; PLUS MAX CONTINUOUS PWR TO CLB THE LAST 800 FT. THE PLANE HELD THE SPD AND ALT USING 'MAX CONTINUOUS' PWR. ABOUT NEWMAN (EL PASO) THE COPLT NOTICED WE WERE STILL IN 'MAX CONTINUOUS' PWR. I ELECTED TO SELECT CRUISE PWR SETTING. WE SETTLED BACK AND WERE ENJOYING A SMOOTH TRIP WHEN WE BEGAN TO FEEL A BUFFET. MUCH TO OUR SURPRISE; WE DISCOVERED THAT THE AIRSPD HAD BLED OFF ABOUT 30 KTS AND WE WERE ACTUALLY 'BEHIND THE PWR CURVE'. MAX PWR DID NOT IMMEDIATELY HELP; SO I DISCONNECTED THE AUTOPLT; MAINTAINED MAX PWR AND BEGAN A DSCNT. DECLARING AN EMER SO WE COULD DSND. WE LEVELED AT 37000 FT AIRSPD; ETC WAS NORMAL; HAVING RETURNED BY 39500 FT. REST OF FLT NORMAL. LOOKING BACK; I WAS OUT OF THE ENVELOPE FOR THAT ALT AND PWR SETTING. NEVER AGAIN WILL I ATTEMPT SUCH AN ALT VS. HEAVY WT. I WILL ALSO PAY MORE ATTN TO THE FMS. THE PLANE IS VERY HONEST AND FORGIVING BUT CANNOT BE MISTREATED.